A Study On The Adsorption Performance Of Seawater Bromide Ion Using Three Chitosans

As one of important chemical raw materials, bromine and its derivatives have played a significant role in national economy and scientific development. Direct bromine production by present manufacturing techniques is limited by the lower Br^- concentration of seawater, so it is necessary to pre-concentrate Br^- from seawater by adsorption. This article studys on the adsorption performance of seawater Br^- using three chitosans, and analysis of the feasibility of bromine water pre-enrichment.In this article, determination methods of Br^- in seawater, dynamic of spectrophotometry and H₂O₂ oxidation - extraction spectrophotometry are compared. Considering the determination results'accuracy, the operation difficulty degree as well as avoids using dangerous chemicals and so on, dynamic spectrophotometry surpasses the H₂O₂ oxidation - extraction spectrophotometry.This article studies the adsorption properties of chitosan as an adsorbent to remove Br^- from bromine solution(100mg·L^-1) by static adsorption test. To obtain the best adsorption conditions, experiments are carried out as function of the degree of deacetylation, contact time, chitosan dosage,pH value, and temperature. Then we determines the adsorption capacity, and adsorption type according to adsorption isotherm and test the effect using chitosan to adsorb bromide ion from seawater. The result has shown that the best conditions for chitosan to remove Br^- are as follows: degree of deacetylation is 95%, the ratio of adsorbent and solution was 10g·L^-1 (m/v), stirring time is 20min, pH is 6.0, temperature is 20℃. And the adsorption process fit well to Langmuir model. Under the optimum adsorption conditions, the adsorption rates of aqueous solution(100mg·L^-1)and seawater samples (53.54mg·L^-1)are 17.20% and 13.75%, respectively. The maximum capacity is 2.954mg·g^-1 of aqueous solution(100mg·L^-1).Orthogonal test method is used to optimize the preparation conditions of lanthanum nitrate chitosan modified by lanthanum nitrate. To get the best adsorption conditions, we preliminarily investigate the effects of chitosan dosage, contact time, pH value, and temperature, determine the adsorption capacity and adsorption type according to adsorption isotherm,and then test the effect of adsorption of Br^- in seawater. Finally, the effect of co-existed anions or higher ionic strength in seawater on adsorption is analyzed. The result shows that the optimal conditions to prepare chitosan lanthanum-modified chitosan are as follows: degree of deacetylation of chitosan is 85%, the dosage is 0.4g, the dosage of lanthanum nitrate is 2g, (the rate of Chitosan to lanthanum nitrate is 1 : 5), stirring time is 8 h. The best adsorption conditions for chitosan modified by lanthanum nitrate are as following: the ratio of adsorbent and solution was 20g·L^-1 (m/v), stirring time is 20min, the solution pH value is 5.0, adsorption temperature is 10℃ 20℃according to the actual situation. Both Langmuir and Freundlich models agree very well with the experimental data for lanthanum-modified chitosan . The adsorption rates of aqueous solution(100mg·L^-1)and seawater samples (53.54mg·L^-1)are 67.78% and 24.99%, and the maximum capacity is 8.396mg·g^-1.The result also shows that a large number of Cl- and SO42- ions in seawater impact the adsorption process significantly, and the high ionic strength of sea water cause loss of lanthanum ion. So that the binding site and adsorption rates reduce.Based on the above questions, it is considered to deal with the chitosan modified lanthanum nitrate.The cross-linking method is used to increase the active groups on adsorbent, and improve the efficiency of absorption. Orthogonal test is used to optimize the preparation conditions. To get the best adsorption conditions, we preliminarily discusses the effects of chitosan dosage, contact time, pH value, and temperature on adsorption, and then determine the adsorption capacity and adsorption type according to adsorption isotherm. The result shows that the optimal conditions to prepare of lanthanum nitrate modified chitosan beads are as follows: chitosan 2g dissolves in 100ml 2% acetic acid solution, then the mixture drops into 1g.L^-1 sodium polyphosphate solution and cross-linked into a ball .Wash the ball with double distilled water to neutral, then put into the mixture of 50ml 1mol.L^-1 NaOH solution and 0.1ml epichlorohydrin, react 2 h under 70℃. Wash the ball with double distilled water to neutral. Put into 50 ml La3+ concentration of 0.25 mol.L^-1 solution and react 10 h .Freeze-drying after cleaning. The best adsorption conditions for lanthanum nitrate modified chitosan beads to remove bromide ion from seawater are as follows: the ratio of adsorbent and seawater was 8g·L^-1 (m/v), stirring time is 40min, pH value is 7.0, and adsorption temperature is 10℃ 20℃according to the actual situation. The Langmuir model agrees very well with the experimental data for lanthanum-modified chitosan beads. The adsorption rate of seawater samples is 32.53%, and the maximum capacity is 2.3403mg·g^-1. Through infrared spectroscopy, it could be concluded that compared with the first two adsorbents, -C-O stretching vibration characteristic peaks of microspheres after cross-linking show a more broad absorption peak at 1083.7cm^-1, This is maily because that the number of hydroxyl and lanthanide ions increased.which is conducive to improve the adsorption rate of Br^- in seawater by cross-linked beads.The study on the adsorption of bromine from seawater by chitosan is still in its initial stage, and it is not reported in literatures at home and abroad. This research studies the adsorption capabilities of three adsorbents to remove Br^- from seawater. The results have showed that: all the three kinds of adsorbents manifest sorption of Br^-, especially lanthanum-modified chitosan beads, which behaves the best adsorption effect, and exhibits application foreground.